1. Field of the Invention
The present invention relates to a controller area network having a flexible data rate.
2. Description of the Related Art
The acceptance and introduction of serial communication to more and more applications has led to requirements that the bandwidth for the serial communication needs to be increased.
A widely used protocol for serial communication is the CAN protocol as specified in the BOSCH CAN Specification 2.0, which can be downloaded from the Robert Bosch GmbH website: http://www.semiconductors.bosch.de, and in the international Standard ISO 11898-1. This CAN protocol is referred to in the following as “Norm CAN”.
Two factors limit the effective data-rate in CAN networks, first the minimum bit time required for the function of the CAN bus arbitration method and second the relation between the numbers of data bits and frame bits in a CAN message.
Recently a new protocol has been developed, that is called “CAN with Flexible Data-Rate” or CAN FD. It still uses the CAN bus arbitration method, it increases the bit-rate by switching to a shorter bit time after the end of the arbitration process and returns to the longer bit time at the CRC Delimiter, before the receivers send their acknowledge bits. The effective data-rate is increased by allowing longer data fields. CAN uses four bits as Data Length Code resulting in 16 different codes, but only the first nine values are used, codes [0-8] standing for data field length of [0-8] bytes. In CAN, the codes [9-15] are defined to signify eight data bytes. In CAN FD, the codes are used to signify longer data fields.
The CAN FD protocol is described in a protocol specification titled “CAN with Flexible Data-Rate Specification”, in the following referred to as the CAN FD Specification. Version 1.0 was released Apr. 17, 2012 and made available to the public for download at the Robert Bosch GmbH website: http://www.semiconductors.bosch.de
As long as unmodified Norm CAN controllers are used, a mixed network of Norm CAN nodes and CAN FD nodes can only communicate in Norm CAN format. That is, all nodes in the network must have a CAN FD protocol controller for CAN FD communication, but all CAN FD protocol controllers are also able to take part in Norm CAN communication. If the CAN FD communication is limited to data fields with a length of up to eight data bytes, it is not necessary to change the application program apart from the initial configuration of the controller.